1. Field of the Invention
Our invention relates to electroplating or electroforming apparatus, and more specifically to such apparatus for producing metal foil, for particular use in the manufacture of printed electronic circuitry, or printed circuit patterns by electrodepositing copper or other metal on a continuously advancing strip of electrically conductive material. Still more specifically our invention concerns improvements in or relating to continuous electroplating or electroforming apparatus of the type described and claimed in U.S. Pat. 4,119,516 issued on Oct. 10, 1978, to Seiichi Yamaguchi and assigned to the assignee of the instant application.
2. Description of the Prior Art
Among the more commonly used processes for the fabrication of printed circuitry is the etched-foil method. This conventional method employs a foil of metal such as copper bonded to a base of electrically insulating material. Over the copper foil is applied an etchant-resist mask in the shape of a desired circuit pattern. The masked copper foil is then placed in an etching solution capable of corroding copper, with the consequent dissolving of the unmasked regions of the copper foil. The etchant-resist mask is then removed by a solvent that attacks only the mask material, leaving the copper circuit pattern on the insulating base.
The manufacture of the metal foil to be processed into printed circuits as above is possible either by mechanical rolling or by electrodeposition. Mechanical rolling imposes limitations, by both technical and economic reasons, on the thickness and width of foil strips that can be produced. Rolled metal foil has a minimum thickness of as much as 35 microns, compared with 17 microns or less attained by electrodeposition. Electroforming of metal foil has therefore almost superseded mechanical rolling.
One well known electroplating apparatus for the production of metal foil employs a rotary drum as a cathode, on which the metal foil is electrodeposited. This conventional electroplating apparatus also has its own problem. Particularly when the metal is electrodeposited on the drum cathode to a thickness of 17 microns or less, the metal foil is easy to develop such imperfections as wrinkles, cracks and fissures while being stripped from the cathode on to a takeup roll. Thus, for the manufacture of metal foil as thin as 5 or 10 microns, for example, it has been customary to electrodeposit the metal on a foil carrier which is itself in the form of a foil of aluminum with a thickness of 40 to 60 microns. This practice is of course highly costly, necessitating wasteful use of aluminum.
The continuous electroplating apparatus of the aforesaid S. Yamaguchi U.S. Pat. No. 4,119,516 is a distinct improvement over the rotary drum-type apparatus as it makes the noted costly practice unnecessary. According to this prior patented apparatus, not only metal foil but also circuit patterns can be continuously electrodeposited on a strip of electrically conductive material traveling horizontally under and in sliding contact with a stationarycathode. It has later proved, however, that the horizontal electroplating apparatus tends to develop pinholes in metal foil or circuit patterns deposited on the conductive strip to a thickness of 17 microns or less. A brief account of this drawback follows.
The electroplating apparatus according to the mentioned U.S. Pat. has a pair of horizontally spaced, insoluble anodes disposed under the cathode, with an interelectrode gap between each anode and the conductive strip traveling under the cathode in sliding contact therewith. Lying between the pair of insoluble anodes is a block of electrically insulating material having formed therein an upwardly opening groove from which the electroplating solution is made to flow turbulently into and through the interelectrode gaps. Desired metal such as copper deposits on the downward-facing surface of the metal strip as same travels over the two insoluble anodes one after the other.
During electroplating operation the apparatus allows leakage of the electric current from the proper plating zones, in proportion to the size of the interelectrode gaps. The leaking current causes some deposition of the metal as coarse crystals on the successive downward-facing surface portions of the conductive strip just before they enter the plating zones.
Then, in the plating zones, the metal crystal tends to grow around the coarse deposits already existing on the conductive strip. The result is the production of pinholes in mesh-like arrangement unless the metal is deposited to a considerable thickness in the plating zones. Such pinholes are a crucial defect in metal foil or circuit patterns as thin as 17 microns or less.